In electrodeposition of organic coatings an object to be painted is immersed in an aqueous bath comprising a dispersion of particles of the film-forming, organic material to be electrodeposited and serve as a first electrode of an electrodeposition cell. A second electrode, which is usually a separate consumable conductor, is immersed in the coating bath and hence is in electrical contact with the bath, and is spaced apart from the object to be coated. These electrodes are in electrical connection with opposite terminals of a direct current power source. A difference of electrical potential above the threshold deposition voltage of the coating material to be deposited, e.g. 50 to 1,000 volts, more commonly between about 100 and about 300 volts, is maintained between the electrodes, a direct current of electrical energy is passed between such electrodes and through the bath, causing the coating material to deposit upon the object to be coated. Such deposition is terminated by the increasing electrical resistance of the resultant coating, removal of the object from the bath, or by any other break in the electrical circuit which comprises the aforementioned electrodes, the electrolyte bath therebetween, the power source and conduction means providing electrical connection between such electrodes and the power source. The coated object is removed from the bath, given a water rinse, and the coating is polymerized, commonly by baking.
In most manufacturing installations where this method of painting is employed, the coating is effected in a continuous process wherein the object to be coated is carried over to the coating bath while suspended from a conveyor from which it may or may not be electrically insulated, an electrical contact is established between the object to be coated and the appropriate lead from the power source (the negative lead in the case of cathodic deposition), the object still supported by the conveyor is immersed in the bath, coated, and then withdrawn from the bath by the conveyor.
Since a continuous stream of such objects pass through the bath to be coated, readily attachable and detachable conveyor hanger connector means are advantageously employed to secure electrical connection between the object to be coated and the proper conductor lead from the power source. With large objects, the electrical current drawn in the coating cycle is necessarily high. Any poor conductor of electric current interposed between the object and the conductor leading to the power source adds resistance, produces heat, and lowers the intended coating voltage. If such resistance is sufficiently high, it can prevent coating altogether. For these reasons, it is important that the detachable connector means be constructed and arranged in a manner such that the area of electrical connection between the object and the conveyor work-carrying conductor or hanger connector through which electrical connection is established with the power source be as free as possible of non-conductive or poorly conductive materials. Maintaining a clean contact area is made difficult by the connecting conductor being of the same polarity as the object to be coated. Those portions of the connecting conductor left exposed to the bath become coated with an insulating film each time the connector is used. In installations where this connecting conductor is not removed from the object until after baking, the problem becomes even greater.
The aforementioned problem of paint build-up on conveyor hangers and the like in electrodeposition painting as well as in electrostatic painting and enameling appears to be relatively well recognized in the art. The unexpired U.S. Pat. Nos. to Witte 4,069,790, Guttman et al 3,830,196, Haney et al 3,830,716, Johnson 3,575,832, Igras et al 3,509,036, and Urquhart 4,263,122 teach various diverse approaches to solving this problem. Witte and Guttman et al both burn off the paint adhering to the supporting element to carbonize or reduce the paint to a coking residue, coupled with a water and/or compressed air flush to blast away such residues from the supporting elements. Haney et al teaches the cleaning of a continuous type electrocoating conveyor belt by a "reverse polarity cleaning" operation requiring, inter alia a second electrode positively charged to a higher positive potential than the conveyor belt. Haney et al also discloses another approach to the problem, namely making the conveyor belt or article support from a material that exhibits a "rectification characteristic", i.e. materials are electrically conductive and will carry a charge to an article carried on the support while at the same time will not themselves become coated. Johnson interposes a conductive foil shield cup between the support so that the shield becomes coated and not the support. Similarly, Igras shields the conductive contact of a support with a resilient shield to prevent the paint from coating the contact during transit through the electrocoating process. Urquhart discloses, inter alia, a trolley clean out obstruction fixed on the rail adjacent the path of travel of the trolley yoke arms and upstream of the grounding shoe for dislodging foreign matter clinging thereto. However, each of these prior approaches suffers from certain disadvantages in terms of equipment, labor and/or material cost, lack of reliability or efficiency in processing, and/or adaptability to various diverse systems.